1. Field of the Invention
The present invention relates to a semiconductor substrate, or an epitaxial substrate, which includes a bulk GaN substrate and an epitaxially grown layer on the bulk GaN substrate, and semiconductor devices implemented by an epitaxial growth on the semiconductor substrate. Here, the semiconductor devices include various semiconductor optical devices such as semiconductor light-emitting devices and various semiconductor electric devices such as transistors.
2. Description of the Related Art
III-V nitride compound semiconductors such as gallium nitride (GaN) have a wide bandgap. Utilizing properties and features of the wide bandgap semiconductors, high-brightness ultraviolet-blue/green light-emitting diodes (LEDs) and blue-violet laser diodes and the like have been studied and developed. Moreover, high frequency/high power III-V nitride compound semiconductor field-effect transistors (FETs) or the like have been fabricated based upon the properties and features of the wide bandgap semiconductors.
Since there were no substrates that allow lattice matching in crystal growth of III-V nitride compound semiconductors in the early stages of research and development, semiconductors devices were fabricated through crystal growth using sapphire (Al2O3), silicon carbide (SiC) or the like as a substrate. A two-stage growth method is used for crystal growth of a III-V nitride compound semiconductor using an Al2O3 substrate. With such a method, however, attributing to differences in lattice constant and thermal expansion coefficient between the Al2O3 and the III-V nitride compound semiconductor, a great number of threading dislocations and defects are generated. These threading dislocations and defects are major problems during practical use in that they bring about a shortened laser diode lifetime, for example, poor reliability.
Therefore, provision of bulk GaN substrates, on which the homoepitaxial growth can be achieved, to a market has been a long-awaited desire. Due to recent advances in crystal growth technology, bulk GaN substrates have become available, and results of fabricating a prototype of an InGaN/GaN/AlGaN-based laser diode have been reported (see S. Nakamura, et al. Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates, Applied Physics Letters, Vol. 72, No. 2, pp. 2014-2016, 20 Apr. 1998).
However, there is little knowledge regarding the bulk GaN substrate itself or homoepitaxial growth on the bulk GaN substrate, and a method for manufacturing a semiconductor device such as a high-performance semiconductor optical device or a high-performance semiconductor electronic device using a III-V nitride compound semiconductor is yet to be established.